In general, to realize high-speed data transmission in a communication system, link aggregation which uses a plurality of communication links as a single large-capacity link has been generally used. Examples of the link aggregation include the IEEE 802.3ad in which link aggregation is performed in a wired Ethernet link (“Ethernet” is a registered trademark).
Furthermore, a technique of efficiently using a link band by assigning data flows to a plurality of links has been generally used. In a configuration in which data flows are assigned to a plurality of links, packets and control messages of the data flows are also transmitted after being assigned to the links, and therefore, time points when the assigned packets arrive at a reception side through the links are different from one another. Therefore, an order of packets is inversed in a data flow in some cases.
To address this problem, sequence numbers are assigned to the transmitted packets after the data flows are identified on a transmission side, the packets are assigned to the plurality of links, and the order is corrected in accordance with the sequence numbers on the reception side. By this, even when the data flows are assigned to the plurality of links, the order of the packets is ensured.
Furthermore, a technique of detecting loss of a packet on a reception side caused by a transmission error and performing a process of transmitting a packet which is in a waiting state when the loss of a packet is detected has been generally used (refer to Japanese Laid-open Patent Publication No. 2004-80139, for example). In this technique, at a timing when a packet having a sequence number which is larger than that of a packet which is expected to be received is received, it is determined that the packet which is expected to be received is lost in transmission. Therefore, in this technique, a reception of a packet is waited until a packet having a sequence number which is larger than that of the packet which is expected to be received is received.
Moreover, a technique of notifying, when a transmission side detects a poor wireless environment in a certain frame, a transmission side of information representing that data is not to be transmitted in the frame has been generally used (refer to Japanese Laid-open Patent Publication No. 2004-7029, for example).